基于MCFT理论的钢纤维混凝土梁的截面分析

根据钢纤维混凝土的特性,对MCFT理论的裂后混凝土平均主应力-平均主应变关系进行了修正。在Vecchio和Collins对钢筋混凝土板在纯剪作用下截面分析的基础上,叠加了弯矩的作用,建立了钢纤维混凝土梁在弯剪复合作用下的截面分析模型。利用作者以及其他研究者的试验对该模型进行了验证,结果表明计算得到的钢纤维混凝土梁的剪力-箍筋应变曲线和极限荷载与实测结果吻合良好。该文还利用该模型对钢纤维和箍筋对梁抗剪性能的影响效率进行了比较。     

Effectiveness of the use of steel fibres on the torsional behaviour of flanged concrete beams

The behaviour under torsion of reinforced concrete beams with steel fibres as mass reinforcement is experimentally investigated. Short hooked-ended steel fibres with aspect ratio l_f/d_f = 37.5 are used. Test results of 35 beams with rectangular, L-shaped and T-shaped cross-sections tested in pure torsion are presented and discussed. Various configurations of conventional and fibre steel reinforcement are examined. The experimental program includes (i) plain concrete beams (control specimens), (ii) specimens with longitudinal reinforcing bars and (iii) specimens with bars and stirrups. All cases are examined with 0%, 1% and 3% steel fibre volume fractions. The use of steel fibres as the only shear torsional reinforcement is also reported herein, in an attempt to examine the effectiveness of fibres as a potential replacement of stirrups. Test results indicated that fibrous concrete beams exhibited improved overall torsional performance with respect to the corresponding non-fibrous control beams. The addition of steel fibres was essential to the tested beams without or with inadequate conventional steel reinforcement. Fibres prevented the sudden brittle failure of both rectangular and non-rectangular beams and proved to be under some circumstances adequate to provide for enhanced torsional moment capacities, even in the case of full replacement of stirrups with steel fibres.     

Shear capacity of steel and polymer fibre reinforced concrete beams

This paper deals with the application of a plasticity model for shear strength estimation of fibre reinforced concrete beams without stirrups. When using plastic theory to shear problems in structural concrete, the so-called effective strengths are introduced, usually determined by calibrating the plastic solutions with tests. This approach is, however, problematic when dealing with fibre reinforced concrete (FRC), as the effective strengths depend also on the type and the amount of fibres. In this paper, it is suggested that the effective tensile strength of FRC can be determined on the basis of the tensile stress-crack opening relationship found from wedge splitting tests. To determine the effective compressive strength of FRC, it is proposed to adopt the formula used for conventional concrete and modify it by introducing a fibre enhancement factor to describe the effect of fibres on the compressive softening behaviour of FRC. The enhancement factor is determined as the ratio of the areas below the stress–strain curves for FRC and for conventional concrete. The outlined approach has been verified by shear testing of beams containing no fibres, 0.5% steel fibre volume and 0.5% polymer fibre volume. The tests results are compared with estimations and show satisfactory agreements, indicating that the proposed approach can be used.     

Response of steel fiber reinforced high strength concrete beams: Experiments and code predictions

The shear-flexure response of steel fiber reinforced concrete (SFRC) beams was investigated.Thirty-six reinforced concrete beams with and without conventional shear reinforcement (stirrups) were tested under a four-point bending configuration to study the effectiveness of steel fibers on shear and flexural strengths, failure mechanisms, crack control, and ductility.The major factors considered were compressive strength (normal strength and high strength concrete up to 100 MPa), shear span-effective depth ratio (a/d = 1.5, 2.5, 3.5), and web reinforcement (none, stirrups and/or steel fibers).The response of RC beams was evaluated based on the results of crack patterns, load at first cracking, ultimate shear capacity, and failure modes.The experimental evidence showed that the addition of steel fibers improves the mechanical response, i.e., flexural and shear strengths and the ductility of the flexural members.Finally, the most recent code-based shear resistance predictions for SFRC beams were considered to discuss their reliability with respect to the experimental findings. The crack pattern predictions are also reviewed based on the major factors that affect the results.     

高延性混凝土加固钢筋混凝土梁受剪性能试验研究及承载力计算

为研究高延性混凝土(HDC)加固钢筋混凝土梁的受剪性能,该文对7根HDC加固梁及4根未加固梁进行静力试验,研究剪跨比、配箍率、加固层厚度和加固层附加箍筋对钢筋混凝土梁破坏形态、荷载-挠度曲线、受剪承载力以及裂缝的影响。结果表明:采用HDC面层对钢筋混凝土梁进行受剪加固,可以显著提高梁的受剪承载力;HDC面层可以代替部分箍筋的受剪作用,改善钢筋混凝土梁的剪切破坏形态;加固试件在达到极限位移之后,试件的完整性较好,剩余承载力较高。基于试验结果,利用桁架-拱模型,提出了HDC加固钢筋混凝土梁的受剪承载力计算公式,计算值与试验值吻合较好。     

Shear Strength Evaluation of Concrete Beams Reinforced with BFRP Bars and Steel fibers without Stirrups

This paper presents experimental and analytical investigations on concrete beams reinforced with basalt fiber reinforced polymer (BFRP) and steel fibers without stirrups. Independent behaviour of BFRP reinforced beams and steel fiber reinforced beams were evaluated and the effect of combining BFRP bars and steel fiber was investigated in detail. It is found that combining steel fibers with BFRP could change the shear failure of BFRP reinforced beam to flexural failure. Further, the existing analytical models were reviewed and compared to predict the shear strength of both FRP reinforced and steel fiber reinforced beams. Based on the review, the appropriate model was chosen and modified to predict the shear strength of BFRP reinforced beam along with steel fibers.     

Shear behaviour of steel fibre reinforced concrete beams

The possibility of substituting traditional transverse reinforcement (stirrups) for steel fibres in precast elements can significantly reduce production costs. In the present paper, the shear behaviour of prestressed elements has been investigated by means of experimental tests on full scale beams. Tests concern beams with conventional as well as steel fibre reinforcement. Experimental results show that the shear behaviour of fibre reinforced concrete beams without conventional reinforcement is similar to, or even better than that of beams with stirrups. When used in beams with stirrups, steel fibres significantly improve their shear strength. A discussion on the contribution of steel fibres on the shear strength is also presented, with reference to the latest RILEM provisions.

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Résumé La possibilité de remplacer des armatures transversales traditionnelles par des fibres d'acier dans des éléments pré-tendus peut apporter des améliorations considérables sur les performances structurales. Dans ce rapport, le comportement en cisaillement des éléments précontraints a été étudié à l'aide de tests expérimentaux sur des poutres à échelle grandeur nature. Les essais portent aussi bien sur des poutres avec armatures traditionnelles que sur des poutres renforcées à l'aide de fibres. Les résultats expérimentaux montrent que les performances des poutres en béton de fibres sans armatures traditionnelles sont similaires, sinon meilleures, à celles des poutres avec armature ordinaire de cisaillement. Lorsqu'elles sont utilisées dans des poutres avec armatures traditionnelles les fibres d'acier améliorent considérablement leur résistance en cisaillement.

     

Effects of stirrup,steel fiber,and beam size on shear behavior of high-strength concrete beams

This study investigates the effectiveness of steel fibers and minimum amount of stirrups on the shear response of various sized reinforced high-strength concrete (HSC) beams. For this, six large reinforced HSC beams with a shear span-to-depth ratio (a/d) of 3.2 were manufactured. Three of them contained 0.75% (by volume) steel fibers without stirrups as per ACI Committee 318, while the rest were reinforced with the minimum amount of stirrups without fibers. Test results indicate that, with increasing beam size, significantly lower shear strength was obtained for steel fiber-reinforced high-strength concrete (SFR-HSC) beams without stirrups, than for the plain HSC beams with stirrups. The inclusion of steel fibers effectively limited crack propagation, produced more diffused initial flexural cracks, and led to higher post-cracking stiffness, compared to plain HSC. On the other hand, the use of minimum stirrups gave better shear cracking behaviors than that of steel fibers, and effectively mitigated the size effect on shear strength. Therefore, a large decrease in shear strength, with an increase in the beam size, was only obtained for SFR-HSC beams without stirrups. A shear strength decrease of 129% was obtained by increasing the effective depth from 181 mm to 887 mm. The shear strengths of reinforced steel fiber-reinforced concrete beams were not accurately predicted by most previous prediction models. Therefore, a new shear strength formula, based on a larger dataset, that considers the size effect, is required.     

Shear-stress transfer across a crack in steel fibre-reinforced concrete

The ability of cracked reinforced concrete to transfer shear stresses is of major importance for concrete members designed to sustain high shear forces. Thereby, the maximum shear capacity is mainly affected by the aggregate interlock mechanism, the dowel action of longitudinal reinforcement, the restraining action of stressed reinforcement crossing the crack interface and the possible presence of stirrups. In case of steel fibre reinforced concrete (SFRC), where fibres are used to replace either completely or partially traditional stirrups, research has proven that the direct shear transfer capacity of cracked concrete is increased significantly by using fibres. By means of 69 direct shear tests, the shear-friction behaviour of SFRC with or without confining pressure has been studied further and existing empirical formulations have been checked. Since these models only provide a maximum shear strength, a more fundamental approach to model the direct shear behaviour of cracked SFRC is proposed in this paper. This model deals with the fibre-matrix interaction by means of fibre pull-out and aggregate interlock, as a function of the shear crack opening behaviour (i.e. combined opening and slipping).     

Effective strain of RC beams strengthened in shear with FRP

The failure modes of Reinforced Concrete (RC) beams strengthened in shear with Fiber Reinforced Polymer (FRP) sheets or strips are not well understood as much as those of RC beams reinforced with steel stirrups. When the beams are strengthened in shear with FRP composites, beams may fail due to crushing of the concrete before the FRP reaches its rupture strain. Therefore, the effective strain of the FRP plays an important role in predicting the shear strength of such beams. This paper presents the results of an analytical and experimental study on the performance of reinforced concrete beams strengthened in shear with FRP composites and internally reinforced with conventional steel stirrups. Ten RC beams strengthened with varying FRP reinforcement ratio, the type of fiber material (carbon or glass) and configuration (continuous sheets or strips) were tested. Comparisons between the observed and calculated effective strains of the FRP in the tested beams failing in shear showed reasonable agreement.     

Flexural analysis of reinforced concrete beams strengthened with a cement based high strength composite material

The structural behaviour of reinforced concrete beams strengthened with a system made by fibre nets embedded into an inorganic stabilized cementitious matrix named Fibre Reinforced Cementitious Mortars (FRCM), was investigated in this paper. The main issues focussed in the paper are: (i) the strengthening effect of the FRCM system on the flexural behaviour of reinforced concrete beams in terms of both ultimate capacity, deflections and ductility and (ii) the influence of the fibre reinforcement ratio on the occurrence of premature failure modes.The analysis refers to a FRCM system made by ultra-high strength fibre meshes such as the Polypara-phenylene-benzo-bisthiazole (PBO) fibres; PBO fibres have, in fact, great impact tolerance, energy absorption capacity superior than the other kind of fibres and chemical compatibility with the cementitious mortar.A total of 12 reinforced concrete beams strengthened in flexure with the PBO-FRCM system have been tested. The influence of some mechanical and geometrical parameters on the structural behaviour of strengthened beams, is analysed both at serviceability and the ultimate conditions. Results of a comparison between experimental results and theoretical predictions, obtained by models usually adopted for the analysis of FRP strengthened concrete structures, are, also, presented and discussed.     

高延性混凝土无腹筋梁受剪性能试验研究

提出采用高延性混凝土改善梁的抗剪性能和变形能力,设计了8个高延性混凝土梁和3个作为对比试件的混凝土梁,并通过静力试验研究不同剪跨比和配筋率高延性混凝土无腹筋梁的破坏形态和破坏机理。高延性混凝土无腹筋梁的剪切破坏形态有挤压破坏、剪压破坏、弯剪破坏和剪拉破坏。试验结果表明：高延性混凝土梁的剪切破坏均表现出一定的延性,与普通混凝土梁的脆性剪切破坏具有明显不同;高延性混凝土梁的剪切裂缝开展缓慢,说明高延性混凝土良好的拉伸应变硬化和多裂缝开展特性能够有效控制剪切裂缝的发展,防止混凝土压碎剥落,显著提高梁的抗剪性能和耐损伤能力;相比普通混凝土无腹筋梁,高延性混凝土无腹筋梁的受剪承载力和变形能力均有明显提高,表明采用高延性混凝土可以显著改善无腹筋梁的脆性剪切破坏模式;剪跨比和纵筋配筋率对高延性混凝土梁的剪切破坏形态和承载力影响较大,其受剪承载力随剪跨比的增大而降低,随配筋率的增大而有所提高。     

Flexural analysis of reinforced concrete beams strengthened with a cement based high strength composite material

The structural behaviour of reinforced concrete beams strengthened with a system made by fibre nets embedded into an inorganic stabilized cementitious matrix named Fibre Reinforced Cementitious Mortars (FRCM), was investigated in this paper. The main issues focussed in the paper are: (i) the strengthening effect of the FRCM system on the flexural behaviour of reinforced concrete beams in terms of both ultimate capacity, deflections and ductility and (ii) the influence of the fibre reinforcement ratio on the occurrence of premature failure modes.The analysis refers to a FRCM system made by ultra-high strength fibre meshes such as the Polypara-phenylene-benzo-bisthiazole (PBO) fibres; PBO fibres have, in fact, great impact tolerance, energy absorption capacity superior than the other kind of fibres and chemical compatibility with the cementitious mortar.A total of 12 reinforced concrete beams strengthened in flexure with the PBO-FRCM system have been tested. The influence of some mechanical and geometrical parameters on the structural behaviour of strengthened beams, is analysed both at serviceability and the ultimate conditions. Results of a comparison between experimental results and theoretical predictions, obtained by models usually adopted for the analysis of FRP strengthened concrete structures, are, also, presented and discussed.     

Structural response of hyperstatic concrete beams reinforced with GFRP bars: Effect of increasing concrete confinement

The design of concrete structures reinforced with glass fibre reinforced polymer (GFRP) bars is influenced by their reduced stiffness and brittleness. In hyperstatic structures, the methodology used in force analysis depends on the ductility of the structural systems, which in this case, being essentially provided by the concrete, can be potentially increased by confining concrete in critical zones. This paper presents experimental and numerical investigations about the flexural behaviour of continuous beams reinforced with GFRP bars, namely of their service and failure responses, and the effect of increasing concrete confinement in critical cross-sections. A calculation procedure to quantify the confinement effect in beams due to the reduction of the spacing between shear stirrups is first presented. The experimental investigations comprised a comparative study in which two-span concrete beams reinforced with either GFRP or steel bars were tested in bending. In the former, the effect of reducing the shear stirrups spacing was analyzed together with the under- and over-reinforcement at the central support and midspan cross-sections, respectively. The development of a crack hinge in the continuity support zone highlighted the better performance of beams under-reinforced on the top layer with GFRP bars compared to “equivalent” beams reinforced with steel, namely at the resistance level. In addition, the confinement at critical zones increased significantly the strength and ductility. The numerical investigations included the development of non-linear finite element models for all beams tested - numerical results are in good agreement with test data and seem to confirm the confinement effect observed in the experiments.     

Comparison between the short and long term behaviour of fibre reinforced and unreinforced concrete beams

The paper deals with the results of tests on concrete beams with and without steel fibres, reinforced (with high yield flexural steel only) and without any reinforcement loaded statically both long and short term. Tests were carried out for both serviceability and ultimate limit states. Measurements were taken of load, strains, crack width and crack pattern, and deflection. For short term loading the beams were loaded incrementally to failure, whilst for the long term tests were loaded incrementally to their service load which was maintained for 28 days before being increased to failure. From the long term tests the effects of creep can be determined.

A total of 15 beams were tested over 28 days. The fibres were stainless steel with an aspect ratio of 55 and a volume fraction of 1·5%. In all cases the results indicate successively better performances as fibre reinforcement and bar reinforcement were added.     

Are steel fibres able to mitigate or eliminate size effect in shear?

This paper reports some recent results of an experimental campaign on fibre reinforced concrete (FRC) beams under shear loading tested at the University of Brescia: nine full scale beams, having a height varying from 500 to 1,500 mm, were tested for investigating the effect of steel fibres on key-parameters influencing the shear response of concrete members, with special emphasis on size effect. All tested members contained no conventional shear reinforcement and different amounts of steel fibres: 0, 0.64 or 1 % by volume. Results show that a relatively low volume fraction of fibres can significantly increase bearing capacity and ductility. The latter determines visible deflection and prior warning of impending collapse, which is not possible in plain concrete beams (without transverse reinforcement). The size effect issue is substantially limited. Results were compared against the shear formulation incorporated in the final draft of fib Model Code, which can be considered a valuable analytical model for FRC beams under shear loading, even in the case of the three deepest elements herein tested.     

基于材料延性的高延性混凝土无腹筋梁受剪性能试验研究

为探讨材料延性对无腹筋梁受剪性能的影响,根据高延性混凝土设计理论,考虑纤维抗拉强度、长径比和纤维掺量等因素的影响,进行了4种不同配合比高延性混凝土（HDC）的力学性能试验,并设计了7个高延性混凝土（HDC）无腹筋梁和2个混凝土（RC）梁对比试件,通过静力试验研究材料延性对无腹筋梁的破坏形态、承载力和剪切变形能力的影响。试验结果表明：1）4组HDC试件分别达到不同的延性要求,其等效弯曲韧性可达砂浆试件的50倍,极限拉应变可达普通混凝土的90倍;2） HDC无腹筋梁的承载力可达RC梁的2.36倍,剪切变形能力可达RC梁的3倍以上,均发生具有一定延性的剪拉破坏;3）除剪跨比和纵筋配筋率外,HDC无腹筋梁的受剪承载力和变形能力均随材料延性的提高而增大,在设计中应予以考虑,并可根据工程实际需要选择相应的材料延性需求。     

高延性混凝土加固RC梁抗剪性能试验研究

为利用高延性混凝土（HDC）良好的拉伸和剪切变形能力,提高无腹筋钢筋混凝土梁的受剪性能,该文通过对9根HDC加固梁、1根高性能复合砂浆加固梁及3根未加固梁进行静力试验,研究剪跨比、加固层厚度和加固层是否配置箍筋对梁破坏形态、受剪承载力及变形能力的影响。结果表明：采用HDC面层对无腹筋梁进行抗剪加固,可以显著提高梁的抗剪承载力和变形能力;HDC面层可代替部分箍筋的抗剪作用,改善无腹筋梁的剪切破坏形态,并提高梁的剪压比限值;HDC加固层越厚,其受剪承载力和变形能力提高越明显,但加固层厚度较大时,需采用措施防止HDC面层间发生剥离破坏;HDC面层配置附加箍筋,可进一步提高试件的受剪承载力和耐损伤能力。基于试验结果,该文提出了HDC加固试件的受剪承载力计算公式,其计算值与试验结果吻合较好。     

外壳预制核心现浇装配整体式RC梁抗剪性能的试验研究

该文以研究外壳预制核心现浇装配整体式钢筋混凝土(RC)梁的抗剪性能为目的,制作了2根新型装配整体式RC梁和1根现浇RC梁,开展了RC梁的抗剪承载力试验,考察新型装配整体式RC梁的荷载-挠度关系、开裂荷载、极限承载力、箍筋应变发展特点及破坏模式,并与现浇RC梁的抗剪性能进行比较,试验研究表明:新型装配整体式RC梁与整浇RC梁具有完全相同的抗剪承载力特性;进一步探讨新型装配式RC梁的极限抗剪承载力计算理论,为装配整体式RC梁的设计提供参考。     

预应力型钢超高强混凝土梁受剪承载力试验研究

基于12榀预应力型钢超高强混凝土简支梁和2榀预应力型钢普通强度混凝土简支梁的受剪试验,揭示了影响试验梁受剪性能的主要因素,探讨了剪跨比、箍筋间距、腹板厚度、混凝土强度和预应力度对试验梁的破坏形态、荷载-挠度曲线、斜截面开裂荷载和受剪承载力的影响规律。试验结果表明：预应力型钢超高强混凝土梁具有更好的受剪承载力和剪切延性,以及更大的刚度;基于试验结果建立了预应力型钢超高强混凝土梁的受剪承载力建议计算公式,计算结果与试验结果吻合较好,表明了该文提出的计算公式具有较高的精度。研究成果将为预应力型钢超高强混凝土梁的设计计算和工程应用提供理论依据。